Optical networks provide a tremendous capacity advantage. Entities wishing to take advantage of the advantages that optical networks offer, must usually make a decision based on their current needs (which may be modest and predictable) and their future needs (which are typically unpredictable). An entity may decide to acquire a network to meet its short-term needs because of it's present financial constraints.
However, this approach carries a risk that the network will be insufficient and may cost more in the long run because the entire network has to be replaced due to inadequacies of the network. Also, any upgrades may require the network to be shut down prior to the upgrade. Such a shut down is costly since no service can be provided, which in turn shuts down a revenue stream. In an industry such as telecommunications, shut down can be extremely costly.
Another approach is to project a long-term need and acquire a network with capabilities to meet the long-term need. This approach also carries inherent risks as well. In the short run, the investment in the network will be wasted to the extent that there will be excess capacity. In the long run, the needs of the entity may change in a different direction and the acquired network will not be able to handle the changed needs efficiently.
Current optical networks typically include wavelength selective switches (WSS) and routers (WSR). A WSS/WSR of degree N is an apparatus which switches wavelengths from N input fibers to N output fibers. Most practical applications require only switching N inputs to (N−1) outputs. The switching/routing operation occurs entirely within the optical domain, although the control signals for the WSS are electrical.
FIG. 1 illustrates a conventional design for a WSS/WSR apparatus 100. In this instance the WSS/WSR apparatus 100 is a degree 4 WSS/WSR. The WSS/WSR apparatus 100 includes a plurality of optical splitters 102, a plurality of wavelength filters 104, and a plurality of optical combiners 106.
Each optical splitter 102 is a 1:4 splitter (one input, four outputs) and each optical combiner is a 4:1 combiner (four inputs, one output). If fully connected, then there may be as many as sixteen wavelength filters 104 (combination of 4 inputs and 4 outputs). However, for simplicity, only the connections to the first optical combiner 106 are illustrated.
A WSS/WSR of degree N with capability to drop wavelengths from any of the N inputs and add wavelengths to any of the N outputs is an OADM of degree N. The simplest and most common type of OADM is a degree-2 OADM.
FIG. 2 illustrates a conventional OADM 200. In this instance, the OADM 200 is a degree 2 OADM. The OADM 200 includes first and second optical splitters 202 and 204, first and second optical combiners 206 and 208, first and second wavelength filters 210 and 212, and first and second wavelength demux/mux devices 214 and 216.
As shown, optical signal sources FROM-WEST and FROM-EAST are connected to the input of the first and second optical splitters 202, 204, respectively. Each optical splitter 202, 204 splits the optical signals and directs the signals to the respective wavelength filters 210, 212 and to the respective wavelength demux/mux devices 214, 216. The first and second optical combiners 206, 208 receive optical signals from the respective wavelength filters 210, 212 and from the respective wavelength demux/mux devices 214, 216 and output the combined optical signals to the TO-EAST and TO-WEST optical signal destinations, respectively.
As noted previously, WSS/WSR may be constructed from one or more OADMs. Indeed, an OADM providing connectivity between more than two fibers is considered to be a WSS or WSR. Typically, network connectivity evolves from network elements—such as OADM/WSS/WSR—from a lower degree (degree 2 being the most common) to a higher degree.
However, if the conventional OADM 200 as shown in FIG. 2 is to be upgraded to a higher degree OADM/WSS/WSR, it is clear that disruption of the signals traversing the OADM 200 will occur since any upgrade will require the OADM 200 to be shut down. In other words, an in-service upgrade, where disruption of service does not occur, cannot take place.
An approach is desired where the system deployed is extremely flexible so that future demands on the optical networks, not yet foreseen, may be handled with ease. As the capacity demand grows and changes, it is desirable to provide a flexible system that can meet the increased demand and the type of demand changes. It is also desirable to have the capability to recover previously inaccessible capacity and without service disruptions.